Not Applicable.
The present invention relates generally to composite articles formed from expanded polytetrafluoroethylene (xe2x80x9cePTFExe2x80x9d) materials, and particularly to a composite article made up of a plurality of polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d) components having differing expansion characteristics.
Conventional micro porous ePTFE articles are utilized for many useful articles, such as filters, fabrics, gaskets, electrical insulation and human implant devices. These ePTFE articles are typically produced by blending PTFE resin with a lubricant, compressing the blended resin into a billet, extruding the billet into an extrudate, drying the extrudate, calendering the extrudate (if desired), stretching or expanding the extrudate, and sintering the expanded extrudate to form the final article. The ePTFE article can be manufactured in any extruded shape, including sheets, tubes, rods or filaments.
The micro porous structure of known ePTFE articles is characterized by a plurality of nodes that are connected together by a plurality of fibrils. The nodes are essentially solid PTFE, having a density of about 2.0-2.2 grams per cubic centimeter, whereas the density of the expanded material is less than about 2.0 grams per cubic centimeter. Typically, the fibrils of ePTFE materials average in length from about 0.1 microns to about 100 microns, and the volume of the structure occupied by the fibrils has a density ranging from about 2.0 grams per cubic centimeter to less than 0.5 grams per cubic centimeter. The shape, size and orientation of the nodes and fibrils within the structure can be controlled by varying the expansion rate, expansion ratio, number of expansion axes and other processing parameters to yield many different structures. It is also known that properties such as the expandability and microstructure of the expanded article vary with the molecular weight, particle size and other physical characteristics of the PTFE resin. Various methods of producing ePTFE with differing physical properties are described in the literature, e.g. U.S. Pat. No. 3,953,566, to Gore, herein incorporated by reference.
For some applications it is desirable to provide an article having an asymmetric microstructure in which the fibrils adjacent one surface of the article are relatively short in comparison to the fibrils disposed adjacent the opposite surface of the article. However, attempted solutions to this technical problem have been unsatisfactory due to cost and functionability considerations.
It is an object of the present invention to overcome the technical problems found in conventional ePTFE articles exhibiting asymmetric microstructures, and to provide a composite material that is made up of a plurality of PTFE components having differing expansion characteristics and that possesses an advantageous asymmetric microstructure.
In accordance with these and other objects of the present invention, a multi-layered article, such as a sheet, film or tube, of ePTFE is provided which includes at least two different component layers. At least one of the layers includes a node-fibril structure presenting substantially full-density nodes that are connected together by a first plurality of relatively short fibrils. At least one additional layer of the article includes a plurality of aggregates connected together by a plurality of relatively long fibrils having an average length of greater than about 100 microns. Each of the aggregates are formed of ePTFE including substantially full density nodes that are connected together by a second plurality of relatively short fibrils. Preferably, the short fibrils have an average length of 10-100 microns. A method of producing the article includes the steps of extruding a first PTFE material to form a first extrudate, and extruding a second PTFE material to form a second extrudate, wherein the first PTFE material is highly expandable relative to the second PTFE material. A stack is formed from at least one layer of the first extrudate and at least one layer of the second extrudate, and the stack is expanded and sintered to form the article.
The article of the present invention includes an asymmetric structure in which the lengths of the fibrils provided in the structure are longer adjacent one surface of the article than adjacent the opposite surface. Numerous advantages are realized from this construction. For example, by providing a material comprised of at least one layer of a first PTFE material of a relatively high expandability, and at least one layer of a second PTFE material of a relatively low expandability, it is possible to provide an article having an asymmetric microstructure that varies in fibril length across the thickness of the article. As such, the article has application as a biomedical material, a filter architecture, a textile fabric, a biotech cell-culture substrate, and the like.